This invention relates to automotive diesel internal combustion engines and, more particularly, to improvements in the design of the pistons and piston rings therefor for the purpose of reducing the rate of lubricating oil consumption and deterioration, both initially and over extended periods of operation.
Conventional pistons used in automotive diesel engines generally are provided with two compression rings and one oil ring. Such a piston is disclosed in U.S. Pat. No. 3,463,057 to Packard et al, which is further provided with an exaggerated clearance between the top land of the piston and the cylinder wall. In some cases where the Packard et al. piston has been used, dramatic improvements in oil consumption and deterioration have been noted. This is believed to be due to the elimination of hard carbon deposits on the top land which apparently prevent the combustion gas pressure from communicating with the upper side of the top ring and sealing it against the bottom side of its groove in the piston and against the cylinder wall. It has been noted, however, that even when conventional pistons are provided with this exaggerated clearance, acceptable oil control does not result in all cases. Moreover, even when the initial rate of oil consumption is acceptable it may increase, in some cases drastically, as a function of operating time.
It is believed that with conventional two compression ring pistons, the gas pressure below the top compression ring exceeds the pressure above the top ring during a portion of the expansion cycle. This pressure inversion causes the top ring to be lifted or blown off its sealing surface, i.e., the bottom side of the piston top ring groove, allowing trapped oil and combustion gas to flow upward around the back of the ring onto the top land of the piston which causes the formation of hard carbon deposits thereon and unnecessary oil consumption. The top land carbon deposits then produce further increases in the oil consumption rate as the operating time increases.
Accordingly, it is the object of the invention described and claimed herein to provide an automotive diesel engine with an improved piston and piston ring assembly wherein the seating behavior of the top ring is markedly improved by the substantial elimination of the above-mentioned pressure inversion for the purpose of improving lubricating oil control initially and over a long period of operation.
This and other objects of the invention as will hereinafter become apparent, are specifically met in an automotive diesel engine having otherwise conventional pistons, including a moderate degree of top land setback, carrying two compression rings and one oil control ring wherein the second compression ring is provided with a cold end clearance or compressed ring gap greatly exaggerated compared to the cold end clearance of the top compression ring, preferably in a ratio of at least 3.0 to 1. The annulus defined by the compression rings, the piston second land, and the cylinder wall has an empirically determined volume as will be explained hereinafter, preferably being within the range of about 0.12 cubic inches to about 0.35 cubic inches. The invention has particular application to turbocharged diesel engines although it is applicable to naturally aspirated engines as well.
In the past, others have provided diesel engine pistons with a circumferential groove between the top and second compression ring. Such is illustrated in Zurner U.S. Pat. No. 3,738,231. A similar groove having a volume within the above-stated limits may be found commercially in pistons used in the 3406 DI-T diesel engine manufactured by the Caterpillar Tractor Company. Whatever the purpose of these grooves, laboratory testing has indicated that some improvement in the amount of hard carbon deposit formation on the piston top land and in oil control can be obtained with a groove of appropriate volume in a piston having a moderate amount of top land setback.
To Applicant's knowledge, no one has previously provided a diesel piston with a ring belt having two compression rings wherein the cold end clearance of the second ring is considerably exaggerated compared to the cold end clearance of the top ring. Such represents a major departure from previous practice wherein engine designers for years have strived to minimize ring end clearances in an effort to cut down on gas leakage. The second ring minimum end clearance of the aforementioned Caterpillar 3406 DI-T engine is about 1.66 times the top ring minimum end clearance which is the largest ratio of which Applicant is aware. Generally speaking, however, the industry practice has been to provide second ring minimum end clearance in the range of 0.8 to 1.5 times the minimum top ring end clearance. Indeed, in Standard No. SAE J929, "Piston Rings and Pistons" recommended by the Society of Automotive Engineers (1977 SAE Handbook), no differentiation is made between recommended end clearances for top and second compression rings. Testing has shown, however, that if the second ring cold end clearance is greatly enlarged compared to cold end clearance of the top ring, some improvement in top land hard carbon deposit formation can be obtained.
It is, however, in the application of both of these design modifications, that is, an empirically optimized second land annulus volume and an exaggerated second ring cold end clearance, to a conventional two compression ring piston having a moderate amount of top ring setback that a dramatic improvement in control of top land hard carbon deposits and in oil control can be obtained which is considerably greater than the combined results of employing these parameters separately. Top land hard carbon deposit formation has been all but eliminated and initial lubricating oil consumption has been significantly reduced. Additionally, whereas many engines employing conventional piston assemblies experience significant increases in lubricating oil consumption as a function of operating time, engines employing this invention experience virtually no significant deterioration in the rate of oil consumption over periods of operation as long as 5000 hours. Further, the degree of lubricating oil contamination is significantly reduced, thereby allowing the extension of the interval between changes of the lubricating oil. Additionally, wearing of the second ring groove bottom side due to pounding is reduced by such an amount that the intervals between overhauls of some engines, particularly those wherein the second ring groove of the piston is machined in the aluminum alloy piston rather than in a cast iron insert, are substantially increased. Also wear of the top side of the top ring is virtually eliminated.